132 ALASKAN GLACIER STUDIES
its surface is not greatly crevassed. The most crevassed part is in a marginal area just east of the minatak where the conditions that cause crevassing will be explained later.
The northern arm, on the other hand, is very vigorous and is made by the union of at least three good-sized tributaries, and probably more. Two of these are visible from the crest of the nunatak, and a third is seen from the mountain side southwest of it. They are also indicated by the two prominent medial moraines which sweep down the central portion of this glacier, and the probability of other tributaries is inferred from the fact that far up the valley there are five medial moraine ribbons. Of the three large visible tributaries which combine to make this arm the southernmost is apparently a through glacier, like the south arm, leading over to the Alsek valley, across a broad, flat, snow-covered divide from which the ice flows in both directions. The central tributary of the north arm comes down as a winding valley glacier of large size from among a distant group of high mountains that lie to the northeast. From the sea to the head of this valley cannot be less than 15 or 20 miles. The northern tributary, which is apparently much the largest and most active of the three, comes from the northwest, and all but the very lower portion is hidden from view behind the mountain wall of the north side of the Nunatak valley. There are some very lofty mountains here, and doubtless numerous tributaries descend from them. This arm of the Nunatak Glacier reaches back to the sources of the Hubbard Glacier as a through glacier.
Thus, while we do not know even the general features of the upper part of the Nunatafc Glacier, we do know that it is a long glacier fed by several large contributing glaciers, and that it must rank as one of the largest glaciers of the region. That the glacier is actively moving is inferred from its profoundly-crevassed condition from its sea face (PI. LIX) as far up the northern arm and its tributaries as we can see. It is, however, much smaller and less active than the Hubbard Glacier. The surface slope is moderate and the snow line fully ten miles from the sea, so that a very large area is exposed to ablation, but not enough to cover the lower end of the sea tongue with moraine. The snow line seems to lie higher on this glacier than on the Hubbard, probably because of the sweep which the ocean winds have across the two broad gaps leading toward the Alsek valley.
There is a pronounced lateral moraine on the north side of the northern arm, and another on the south side just above the junction of the north and south arms, which contributes to the medial moraine already mentioned. There are also two winding medial moraines which are pushed first to the east by the incoming of the northwestern tributary, then to the west by the incoming of the eastern tributary; then, after flowing medially down nearly to the junction of the two arms, these moraines are again turned at right angles toward the west and thence they extend to the sea (PI. LX, A) near and parallel to the medial moraine formed by the junction of the north and, south arms. The two medial moraines of the north arm are gray in color, while that made by the union of the two arms is black, suggesting that the mountain at whose base it is formed is made of black hornblende gneiss. That the northern arm is far the stronger of the two is certain from the course pursued by the three medial moraines, which show the ice currents. While they are turning westward they are also extending out across the southern arm; and the medial moraine made by the union of the two anna reaches a point nearly opposite the center of this arm before it is completely turned. It enters the sea in the southern quarter of the glacier. , We, therefore, infer that, at most, the south arm of